Method of fabricating a semiconductor thin film

ABSTRACT

A crystalline semiconductor having an even surface and a large crystal grain size is formed on an economical glass substrate using a laser crystallizing technology. A series of processes, including forming an insulation film on a glass substrate; forming a semiconductor film in the first layer; crystallizing the semiconductor film in the first layer by irradiating laser light stepwise from weak energy laser light to strong energy laser light; forming a semiconductor film in a second layer having a film thickness thinner than that of the semiconductor film in the first layer; performing laser crystallization of the semiconductor thin film in the second layer by irradiating laser light stepwise from weak energy laser light to strong energy laser light, are continuously performed without exposing the workpiece to the atmosphere.

This is a division of application Ser. No. 09/959,644, filed Nov. 2,2001, the entire disclosure of which is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor thin film that isfabricated by a laser crystallizing method, as well as a thin filmtransistor and semiconductor devices, such as a liquid crystal displayapparatus, an active matrix type liquid crystal apparatus, a solar celland the like, which use the semiconductor thin film, and a method offabricating the semiconductor thin film, the thin film transistor andthe semiconductor devices.

The laser crystallizing technology is most widely viewed as a means tofabricate a crystalline semiconductor at a low cost, which crystallinesemiconductor is of the type which is applied to a high performance thinfilm transistor, a high value-added liquid crystal display apparatus anda solar cell. Because the crystallization of a semiconductor thin filmlocally heats only the vicinity of the semiconductor surface by laserirradiation, a low-cost glass substrate and a low-cost organic resinsubstrate can be used for the supporting substrate, and, accordingly,the laser crystallizing technology contributes to a reduced cost. Sincethe laser-irradiated semiconductor is first liquefied and thensolidified so as to be crystallized, a high-quality crystallinesemiconductor having less defects can be obtained. One way to improvethe film quality of the crystalline semiconductor is to increase thecrystal grain size. By increasing the crystal grain size, the volumetricratio of the crystal grain boundary, including defects, to the wholesemiconductor film is decreased, and, consequently, the mobility ofelectrons and holes is improved. Further, a decrease itself in number ofthe defects improves the quality of the crystalline semiconductor. Inregard to a way to increase the crystal grain size, (1) Dig. Of Tech.Papers, 1997, Int. Workshop Active Matrix Liquid Crystal Display(Business Center of Academic Societies, Tokyo 1997), p59, proposes amethod in which, after crystallizing an amorphous silicon by laserirradiation, an amorphous silicon film is formed on the fabricatedpolycrystalline silicon, and then the amorphous silicon is crystallizedby laser irradiation.

On the other hand, a problem of the crystalline semiconductor, forexample, the polycrystalline silicon fabricated by a laser crystallizingmethod, concerns the formation of an uneven surface due to manyprojections which are produced when a high-quality polycrystallinesilicon having large crystal grain size is fabricated. The height of theprojections is nearly equal to the film thickness of the semiconductorbefore irradiating the laser light. A mechanism for producing suchprojection is considered in the Applied Physics Letters, Vol.68, No.15,1996, p2138, which indicates that the projections are formed byvolumetric expansion caused by the phase transition from the meltedsilicon to the solid silicon at a boundary where surfaces of crystalgrowth in a direction lateral to the substrate surface collide with eachother. The crystal growth in the lateral direction generally occurs whena crystal having a crystal grain size larger than the thickness of thesemiconductor thin film is formed. When a semiconductor thin film havinglarge unevenness is used to form the active layer of a coplanar typethin film transistor, the concentration of an electric field occurs atthe projections so as to cause dielectric breakdown in the gateinsulation film serving as the upper layer, of the active layer or tocause reduction of the reliability of the gate insulation film such asby production of defects due to hot carriers. In order to protectagainst these problems, the thickness of the gate insulation film needsto be formed so as to be thicker than 100 nm, and, consequently, itbecomes difficult to drive the thin film transistor with low powerconsumption. Further, since the crystallinity of the projection is verylow and the projection is located in a channel area when thesemiconductor having many projections is used for a coplanar type or anormal stagger type thin film transistor, the ON current is reduced. Inregard to techniques for suppressing the occurrence of the projectionswhen the semiconductor thin film is crystallized with laser light, thefollowing techniques have been reported.

-   (2) A method of irradiating laser light in stages with a pitch of 10    mJ/cm2 is described in IEEE TRANSACTIONS ON ELECTRON DEVICES,    Vol.42, No.2, 1995, p251.-   (3) A method of irradiating laser light after poly-crystallizing    amorphous silicon using a solid phase growth method is described in    Dig. of Tech. Papers 1997, Int. Workshop Active Matrix Liquid    Crystal Displays (Business Center of Academic Societies, Tokyo    1997), p167.-   (4) A method of changing the shape of a laser beam so as to have a    wide lower slopes is described in Shin-etsu Chemical Technical    Report EID98-19 (1998-06), p67.

The above-described conventional technology (1) which proposes toincrease the crystal grain size of the crystalline semiconductor has aproblem in that the crystal grain size can be certainly increased, butprojections having a height nearly equal to the film thickness of thesemiconductor are produced, and, accordingly, a large unevenness isproduced. Further, there is another problem in that, because theamorphous silicon before laser irradiation is exposed to the atmospherein order to perform dehydrogenation and, thereby, a natural oxide filmis formed on the surface, oxygen enters into the silicon film when it iscrystallized by laser light to reduce the quality of the film.

On the other hand, the conventional technology (2) for suppressingproduction of the projections has a problem in that, since laser lightis irradiated in stages with a small pitch of 10 mJ/cm² and the finecrystalline silicon that is first formed is difficult to melt, what canbe fabricated is only polycrystalline silicon having a crystal grainsize of nearly 60 nm, and, accordingly, polycrystalline silicon having alarge crystal grain size above 500 nm can not be fabricated. Theconventional technology (3) for suppressing production of theprojections has a problem in that, because the solid phase growth methodis used and, consequently, the silicon is heated at 1000° C., aneconomical glass substrate can not be used, and, accordingly, thecrystalline semiconductor can not be fabricated with a low cost. Theconventional technology (4) for suppressing production of theprojections has a problem in that, since the crystal grain size becomessmall as the projection is made small, the small roughness and the largecrystal grain size are not compatible with each other.

SUMMARY OF THE INVENTION

An object of the present invention is to increase the crystal grain sizeand suppress the formation of projections on the semiconductor surfaceso as to be compatible with each other.

Another object of the present invention is to provide a flat crystallinesemiconductor of high performance, which is low in cost and which has afilm thickness above 5 nm, an average crystal grain size above 500 nmand an average surface roughness below 5 nm.

In order to solve the above-mentioned problems, a 1st aspect inaccordance with the present invention is directed to a semiconductorthin film which is characterized by the fact that the semiconductor thinfilm is formed on a glass substrate by a laser crystallizing method, andit has a film thickness within a range of 40 nm to 100 nm; an averageroughness of the surface smaller than 5 nm; and an average size of thecrystal grains larger than 500 nm.

A 2nd aspect in accordance with the present invention is directed to asemiconductor thin film which is characterized by the fact that thesemiconductor thin film is formed on a glass substrate by a lasercrystallizing method, and it has a film thickness within a range of 40nm to 100 nm; an average roughness of the surface smaller than 5 nm; andan average size of the crystal grains larger than 500 nm, wherein whenthe surface area of an arbitrary crystal grain is expressed by Sn andthe circumferential length on the surface of the arbitrary crystal grainis expressed by Ln, at least more than 50% of the crystal grains satisfythe relation Ln≦4πRn where Rn=(Sn/π)^(1/2).

A 3rd aspect in accordance with the present invention is directed to asemiconductor thin film which is characterized by the fact that thesemiconductor thin film is formed on a glass substrate by a lasercrystallizing method, and it has a film thickness within a range of 40nm to 100 nm; an average roughness of the surface smaller than 5 nm; andan average size of the crystal grains larger than 500 nm, wherein whenthe surface area of an arbitrary crystal grain is expressed by Sn andthe circumferential length on the surface of the arbitrary crystal grainis expressed by Ln, at least more than 50% of the crystal grains satisfythe relation Ln≦4πRn where Rn=(Sn/π)^(1/2), and further, when crystalstructures on an arbitrary cross section of the crystallinesemiconductor thin film are observed, at least more than 70% of thecrystal grains continuously extend from an interface between thesemiconductor layer and a base layer to the semiconductor surfacewithout discontinuity at some midpoint.

A 4th aspect in accordance with the present invention is directed to asemiconductor thin film which is characterized by the fact that thesemiconductor thin film is formed on a glass substrate by a lasercrystallizing method, and it has a film thickness thicker than 40 nm; anaverage roughness of the surface smaller than 5 nm; and an average sizeof the crystal grains larger than 500 nm.

A 5th aspect in accordance with the present invention is directed to asemiconductor thin film which is characterized by the fact that thesemiconductor thin film is formed on a glass substrate by a lasercrystallizing method, and it has a film thickness thicker than 40 nm; anaverage roughness of the surface smaller than 5 nm; and an average sizeof the crystal grains larger than 500 nm, wherein when the surface areaof an arbitrary crystal grain is expressed by Sn and the circumferentiallength on the surface of the arbitrary crystal grain is expressed by Ln,at least more than 50% of the crystal grains satisfy the relationLn≦4πRn where Rn=(Sn/π))^(1/2).

In addition to the characteristics of the semiconductor thin film of the1st, the 2nd, the 3rd, the 4th or the 5th aspects of the invention, a6th aspect in accordance with the present invention is directed to asemiconductor thin film which is characterized by the fact that theorientation of the semiconductor thin film is mainly in the (1. 1. 1)plane.

In addition to the characteristics of the semiconductor thin film of the1st, the 2nd, the 3rd, the 4th, the 5th or the 6th aspects of theinvention, a 7th aspect in accordance with the present invention isdirected to a semiconductor thin film which is characterized by the factthat the semiconductor is made of silicon.

In addition to the characteristics of the semiconductor thin film of the1st, the 2nd, the 3rd, the 4th, the 5th, the 6th or the 7th aspects ofthe invention, an 8th aspect in accordance with the present invention isdirected to a semiconductor thin film which is characterized by the factthat at least a part of the crystal grain boundaries are positioned, andorientation of that part of the crystal grains is mainly, in the (1. 0.0) plane or (1. 1. 0) plane.

In addition to the characteristics of the semiconductor thin film of the1st, the 2nd, the 3rd, the 4th, the 5th, the 6th, the 7th or 8th aspectsof the invention, 9th aspect in accordance with the present invention isdirected to a semiconductor thin film which is characterized by the factthat the glass substrate is made of a no-alkali glass, which has asoftening point lower than 700° C.

A 10th aspect in accordance with the present invention is directed to amethod of fabricating a semiconductor thin film which is characterizedby the fact that the method comprises the steps of forming an insulationfilm on a glass substrate; forming a semiconductor thin film on theinsulation film; and continuously following the semiconductor thin filmforming with crystallizing of the formed semiconductor thin film byirradiating laser light without exposing it to the atmosphere, whereinthe steps of forming the semiconductor thin film and crystallizing thesemiconductor thin film by irradiating it with laser light are repeatedat least two times, and the irradiating of the laser light in each ofthe steps is such that energy of the laser light is increased in stepsfrom scanning of a weak energy laser light to scanning of a strongenergy laser light.

An 11th aspect in accordance with the present invention is directed to amethod of fabricating a semiconductor thin film which is characterizedby the fact that the method comprises the steps of forming an insulationfilm on a glass substrate; forming a semiconductor thin film on theinsulation film; and continuously following the semiconductor thin filmforming with crystallizing of the formed semiconductor thin film byirradiating laser light without exposing it to the atmosphere, whereinthe step of forming the semiconductor thin film and crystallizing thesemiconductor thin film by irradiating it with laser light are repeatedat least two times, and the thickness of the film in an upper layer atthe time of semiconductor film forming is thinner than thickness of thefilm in a lower layer.

In addition to the characteristic of the fabricating method of the 10thor the 11th aspects of the invention, a 12th aspect in accordance withthe present invention is directed to a method of fabricating asemiconductor thin film which is characterized by the fact that the filmthickness of the semiconductor thin film formed in the first layer iswithin a range of 30 nm to 70 nm, and the film thickness of thesemiconductor thin film formed in the second layer is within a range of25 nm to 40 nm.

In addition to the characteristics of the fabricating method of the10th, the 11th or the 12th aspects of the invention, a 13th aspect inaccordance with the present invention is directed to a method offabricating a semiconductor thin film which is characterized by the factthat the formed semiconductor thin film is silicon having aconcentration of combined hydrogen in the film of less than 10%.

In addition to the characteristics of the fabricating method of the10th, the 11th, the 12th or the 13th aspects of the invention, a 14thaspect in accordance with the present invention is directed to a methodof fabricating a semiconductor thin film which is characterized by thefact that the temperature of the substrate during the lasercrystallization is within a range of 200° C. to 500° C.

In addition to the characteristics of the fabricating method of the10th, the 11th, the 12th, the 13th or the 14th aspects of the invention,a 15th aspect in accordance with the present invention is directed to amethod of fabricating a semiconductor thin film which is characterizedby the fact that the glass substrate used is a no-alkali glass and has asoftening point lower than 700° C.

A 16th aspect in accordance with the present invention is directed to asemiconductor apparatus containing a thin film transistor which ischaracterized by the fact that the thin film transistor is formed on aglass substrate, and a semiconductor thin film formed in an active layerof the thin film transistor through a laser crystallizing method has athickness within a range of 40 nm to 100 nm; an average roughness of thesurface smaller than 5 nm; and an average size of the crystal grainslarger than 500 nm.

A 17th aspect in accordance with the present invention is directed to asemiconductor apparatus containing a thin film transistor which ischaracterized by the fact that the thin film transistor is formed on aglass substrate, and a semiconductor thin film formed in an active layerof the thin film transistor through a laser crystallizing method has athickness within a range of 40 nm to 100 nm; an average roughness of thesurface smaller than 5 nm; and an average size of the crystal grainslarger than 500 nm, wherein when the surface area of an arbitrarycrystal grain is expressed by Sn and the circumferential length on thesurface of the arbitrary crystal grain is expressed by Ln, at least morethan 50% of the crystal grains satisfy the relation Ln≦4πRn whereRn=(Sn/π)^(1/2).

An 18th aspect in accordance with the present invention is directed to asemiconductor apparatus containing a thin film transistor which ischaracterized by the fact that the thin film transistor is formed on aglass substrate, and a semiconductor thin film formed in an active layerof the thin film transistor through a laser crystallizing method has athickness within a range of 40 nm to 100 nm; an average roughness of thesurface smaller than 5 nm; and an average size of the crystal grainslarger than 500 nm, wherein when the surface area of an arbitrarycrystal grain is expressed by Sn and the circumferential length on thesurface of the arbitrary crystal grain is expressed by Ln, at least morethan 50% of the crystal grains satisfy the relation Ln≦4πRn whereRn=(Sn/π)^(1/2), and further, when crystal structures on an arbitrarycross section of the crystalline semiconductor thin film are observed,at least more than 70% of the crystal grains continuously extend from aninterface between the semiconductor layer and a base layer to thesemiconductor surface without a discontinuity at some midpoint.

A 19th aspect in accordance with the present invention is directed to asemiconductor apparatus containing a thin film transistor which ischaracterized by the fact that the thin film transistor is formed on aglass substrate, and the semiconductor thin film of the 4th, the 5th,the 6th, the 7th, the 8th or the 9th aspects of invention is used in anactive layer of the thin film transistor.

A 20th aspect in accordance with the present invention is directed to asemiconductor apparatus containing a coplanar type or a normal staggertype thin film transistor which is characterized by the fact that thethin film transistor is formed on a glass substrate, and thesemiconductor thin film of the 1st, the 2nd, the 3rd, the 4th, the 5th,the 6th, the 7th, the 8th or the 9th aspects of the invention is used inan active layer of the thin film transistor, and the film thickness of agate insulation film of the thin film transistor is thinner than 80 nmor the ratio of the film thickness of the gate insulation film to a filmthickness of the active layer is smaller than 8/6.

A 21st aspect in accordance with the present invention is directed to asemiconductor apparatus containing a coplanar type or a normal staggertype thin film transistor which is characterized by the fact that thethin film transistor is formed on a glass substrate, and thesemiconductor thin film of the 1st, the 2nd, the 3rd, the 4th, the 5th,the 6th, the 7th, the 8th or the 9th aspects of the invention is used inan active layer of the thin film transistor, and the film thickness of agate insulation film of the thin film transistor is thinner than thefilm thickness of the active layer.

In addition to the characteristics of the semiconductor apparatus of the16th, the 17th, the 18th, the 19th the 20th or the 21st aspects of theinvention, a 22nd aspect in accordance with the present invention isdirected to a semiconductor apparatus containing a thin film transistorwhich is characterized by the fact that the glass substrate is made of ano-alkali glass which has a softening point lower than 700° C.

A 23rd aspect in accordance with the present invention is directed to asemiconductor apparatus containing a solar cell which is characterizedby the fact that a semiconductor thin film formed at least in a firstlayer in semiconductor layers of the solar cell through a lasercrystallizing method has a thickness thicker than 40 nm; an averageroughness of the surface smaller than 5 nm; and an average size of thecrystal grains larger than 500 nm.

A 24th aspect in accordance with the present invention is directed to asemiconductor apparatus containing a solar cell which is characterizedby the fact that a semiconductor thin film formed at least in a firstlayer in semiconductor layers of the solar cell through a lasercrystallizing method has a thickness thicker than 40 nm; an averageroughness of the surface smaller than 5 nm; and an average size of thecrystal grains larger than 500 nm, wherein when the surface area of anarbitrary crystal grain is expressed by Sn and the circumferentiallength on the surface of the arbitrary crystal grain is expressed by Ln,at least more than 50% of the crystal grains satisfy the relationLn≦4πRn where Rn=(Sn/π)^(1/2).

A 25th aspect in accordance with the present invention is directed to asemiconductor apparatus containing a solar cell which is characterizedby the fact that a semiconductor thin film formed at least in a firstlayer in semiconductor layers of the solar cell through a lasercrystallizing method has a thickness thicker than 40 nm; an averageroughness of the surface smaller than 5 nm; and an average size of thecrystal grains larger than 500 nm, wherein when the surface area of anarbitrary crystal grain is expressed by Sn and the circumferentiallength on the surface of the arbitrary crystal grain is expressed by Ln,at least more than 50% of the crystal grains satisfy the relationLn≦4πRn where Rn=(Sn/π)^(1/2), and further, when crystal structures onan arbitrary cross section of the crystalline semiconductor thin filmare observed, at least more than 70% of the crystal grains continuouslyextend from an interface between the semiconductor layer and a baselayer to the semiconductor surface without a discontinuity at somemidpoint.

A 26th aspect in accordance with the present invention is directed to asemiconductor apparatus containing a solar cell which is characterizedby the fact that the semiconductor thin film of the 1st, the 2nd, the3rd, the 6th, the 7th, the 8th or the 9th aspects of the invention isused at least in a first layer in semiconductor layers of the solarcell.

A 27th aspect in accordance with the present invention is directed to amethod of fabricating a semiconductor apparatus containing a thin filmtransistor which is characterized by the fact that the method offabricating the semiconductor thin film of the 10th, the 11th, the 12th,the 13th, the 14th or the 15th aspects of the invention is applied tofabrication of an active layer of the thin film transistor.

A 28th aspect in accordance with the present invention is directed to amethod of fabricating a semiconductor apparatus containing a solar cellwhich is characterized by the fact that the method of fabricating thesemiconductor thin film of the 10th, the 11th, the 12th, the 13th, the14th or the 15th aspects of the invention is applied to fabrication ofat least a first layer in semiconductor layers of the solar cell.

A 29th aspect in accordance with the present invention is directed to asemiconductor apparatus containing an active matrix type liquid crystaldisplay apparatus in which a thin film transistor is used as a driveelement in a pixel or a peripheral circuit, wherein the semiconductorapparatus is characterized by the fact that a no-alkali glass having asoftening point lower than 700° C. is used for a supporting substrate,and the thin film transistor of the 16th, the 17th, the 18th, the 19th,the 20th, the 21st or the 22nd aspects of the invention is used for thedrive element in the pixel or the peripheral circuit of the activematrix type liquid crystal display apparatus.

A 30th aspect in accordance with the present invention is directed to amethod of fabricating a semiconductor apparatus containing an activematrix type liquid crystal display apparatus in which a thin filmtransistor is used as a drive element in a pixel or a peripheralcircuit, wherein the method is characterized by the fact that ano-alkali glass having a softening point lower than 700° C. is used fora supporting substrate, and the method of fabricating a thin filmtransistor of the 27th aspects of the invention is applied tofabrication of the thin film transistor of the active matrix type liquidcrystal display apparatus.

A 31st aspect in accordance with the present invention is directed to asemiconductor apparatus containing an active matrix type liquid crystaldisplay apparatus in which a thin film transistor is used as a driveelement in a pixel and one of electrodes of a signal storage capacitorin the pixel is formed of a semiconductor thin film in the same layer asan active layer of the thin film transistor, wherein the semiconductorapparatus is characterized by the fact that a no-alkali glass having asoftening point lower than 700° C. is used for a supporting substrate,and the semiconductor thin film forming the one of the electrodes of asignal storage capacitor in the pixel of the active matrix type liquidcrystal display apparatus has a film thickness within a range of 40 nmto 100 nm; an average roughness of the surface smaller than 5 nm; and anaverage size of the crystal grains larger than 500 nm.

A 32nd aspect in accordance with the present invention is directed to asemiconductor apparatus containing an active matrix type liquid crystaldisplay apparatus in which a thin film transistor is used as a driveelement in a pixel and one of the electrodes of a signal storagecapacitor in the pixel is formed of a semiconductor thin film in thesame layer as an active layer of the thin film transistor, wherein thesemiconductor apparatus is characterized by the fact that a no-alkaliglass having a softening point lower than 700° C. is used for asupporting substrate, and the semiconductor thin film forming the one ofthe electrodes of the signal storage capacitor in the pixel of theactive matrix type liquid crystal display apparatus has a film thicknesswithin a range of 40 nm to 100 nm; an average roughness of the surfacesmaller than 5 nm; and an average size of the crystal grains larger than500 nm, wherein when the surface area of an arbitrary crystal grain isexpressed by Sn and the circumferential length on the surface of thearbitrary crystal grain is expressed by Ln, at least more than 50% ofthe crystal grains satisfy the relation Ln≦4πRn where Rn=(Sn/π)^(1/2).

A 33rd aspect in accordance with the present invention is directed to asemiconductor apparatus containing an active matrix type liquid crystaldisplay apparatus in which a thin film transistor is used as a driveelement in a pixel and one of the electrodes of a signal storagecapacitor in the pixel is formed of a semiconductor thin film in thesame layer as an active layer of the thin film transistor, wherein thesemiconductor apparatus is characterized by the fact that a no-alkaliglass having a softening point lower than 700° C. is used for asupporting substrate, and the semiconductor thin film forming the one ofthe electrodes of the signal storage capacitor in the pixel of theactive matrix type liquid crystal display apparatus has a film thicknesswithin a range of 40 nm to 100 nm; an average roughness of the surfacesmaller than 5 nm; and an average size of the crystal grains larger than500 nm, wherein when the surface area of an arbitrary crystal grain isexpressed by Sn and the circumferential length on the surface of thearbitrary crystal grain is expressed by Ln, at least more than 50% ofthe crystal grains satisfy the relation Ln≦4πRn where Rn=(Sn/π)^(1/2),and further when crystal structures on an arbitrary cross section of thecrystalline semiconductor thin film are observed, at least more than 70%of the crystal grains continuously extend from an interface between thesemiconductor layer and a base layer to the semiconductor surfacewithout a discontinuity at some midpoint.

A 34th aspect in accordance with the present invention is directed to asemiconductor apparatus containing an active matrix type liquid crystaldisplay apparatus in which a thin film transistor is used as a driveelement in a pixel and one of the electrodes of a signal storagecapacitor in the pixel is formed of a semiconductor thin film in thesame layer as an active layer of the thin film transistor, wherein thesemiconductor apparatus is characterized by the fact that a no-alkaliglass having a softening point lower than 700° C. is used for asupporting substrate, and the semiconductor thin film forming the one ofthe electrodes of the signal storage capacitor in the pixel is thesemiconductor thin film of the 4th, the 5th, the 6th, the 7th, the 8thor the 9th aspects of the invention.

A 35th aspect in accordance with the present invention is directed to amethod of fabricating a semiconductor apparatus containing an activematrix type liquid crystal display apparatus in which a thin filmtransistor is used as a drive element in a pixel and one of theelectrodes of a signal storage capacitor in the pixel is formed of asemiconductor thin film in the same layer as an active layer of the thinfilm transistor, wherein the method is characterized by the fact that ano-alkali glass having a softening point lower than 700° C. is used fora supporting substrate, and the method of fabricating the semiconductorthin film of the 10th, the 11th, the 12th, the 13th, the 14th or the15th aspects of the invention is applied to fabrication of thesemiconductor thin film forming the one of electrodes of the signalstorage capacitor in the pixel.

A 36th aspect in accordance with the present invention is directed to amethod of fabricating a semiconductor thin film or a semiconductorapparatus of the 10th, the 11th, the 12th, the 13th, the 14th, the 15th,the 27th, the 28th, the 29th, the 30th or the 35th aspect of theinvention, wherein the method is characterized by the fact that thesemiconductor thin film or the semiconductor apparatus is fabricatedusing an apparatus in which at least a film forming apparatus forforming a semiconductor thin film and a laser crystallizing apparatusare connected to each other by a transfer apparatus having an evacuatingapparatus.

In addition to the characteristics of the method of fabricating thesemiconductor thin film or the semiconductor apparatus of the 10th, the11th, the 12th, the 13th, the 14th, the 15th, the 27th, the 28th the29th, the 30th or 35th aspects of the invention, a 37th aspect inaccordance with the present invention is directed to a method offabricating a semiconductor thin film or a semiconductor apparatus whichis characterized by the fact that the semiconductor thin film or thesemiconductor apparatus is fabricated using an apparatus in which atleast a film forming apparatus for forming a semiconductor thin film, afilm forming apparatus for forming an insulation film and a lasercrystallizing apparatus are connected to each other by transferapparatuses each having an evacuating apparatus.

In addition to the characteristic of the 36th or the 37th aspects of theinvention, a 38th aspect in accordance with the present invention isdirected to a method of fabricating a semiconductor thin film or asemiconductor apparatus which is characterized by the fact that theatmosphere of the transfer apparatus is maintained in a vacuum below10-5 torr or an atmosphere of an inert gas such as nitrogen, helium,neon, argon or the like.

In addition to the characteristics of the 10th, the 11th, the 12th, the13th, the 14th, the 15th, the 27th, the 28th, the 29th, the 30th or the35th aspects of the invention, a 39th aspect in accordance with thepresent invention is directed to a method of fabricating a semiconductorthin film or a semiconductor apparatus which is characterized by thefact that the semiconductor thin film or the semiconductor apparatus isfabricated using an in-line type apparatus in which at least a filmforming portion for forming a semiconductor thin film, a lasercrystallizing portion and a transfer portion are placed in a singlechamber.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1(A) to 1(D) are diagrammatic views illustrating the sectionalstructure and the plane structure of a polycrystalline silicon thin filmfabricated in accordance with the present invention.

FIG. 2 is a graph illustrating method of calculating an averageroughness of a surface when the surface is analyzed by a two-dimensionalsectional structure.

FIGS. 3(A) and 3(B) are schematic diagrams showing a film formingapparatus and a laser crystallizing apparatus, respectively, used in thefabricating process to which the method of fabricating thepolycrystalline silicon thin film in accordance with the presentinvention is applied.

FIGS. 4(A) to 4(E) are cross-sectional diagrams illustrating afabricating process for polycrystalline silicon in accordance with thepresent invention.

FIGS. 5(A) to 5(C) are cross-sectional diagrams showing a fabricatingprocess for the production of a semiconductor thin film in which crystalgrain boundaries are positioned.

FIG. 6 is a schematic diagram showing a semiconductor thin filmfabricating apparatus of an inline type which is used at the time whenthe present invention is applied to fabrication of a semiconductordevice.

FIGS. 7(A) to 7(D) are cross-sectional diagrams showing the fabricatingprocess to which the method of fabricating the polycrystalline siliconthin film in accordance with the present invention is applied.

FIG. 8 is a graph comparing the gate voltage versus drain currentcharacteristics of a polycrystalline silicon thin film transistor inaccordance with the present invention and a thin film transistorfabricated through a conventional fabricating method.

FIG. 9 is a schematic circuit diagram showing the construction of anembodiment of an active matrix type liquid crystal display apparatus inaccordance with the present invention.

FIG. 10 is a diagrammatic view showing the plane structure of one pixelin an embodiment of an active matrix type liquid crystal displayapparatus in accordance with the present invention.

FIG. 11 is a cross-sectional view showing the structure of one pixel inthe embodiment of the active matrix type liquid crystal displayapparatus in accordance with the present invention.

FIG. 12(A) is plan view and FIG. 12(B) is a sectional view of a pixel inan active matrix type liquid crystal display apparatus in which thepolycrystalline silicon in accordance with the present invention is usedfor an electrode on one side of a signal accumulating capacitor in thepixel.

FIGS. 13(A) and 13(B) are layout diagrams showing the construction of aperipheral circuit containing liquid crystal display apparatus to whichthe present invention is applied.

FIGS. 14(A) to 14(C) are cross-sectional is a view showing an example ofa laser irradiation method in accordance with the present invention.

FIGS. 15(A) and 15(B) are cross-sectional views of respectiveembodiments of thin film transistors in a peripheral circuit containingliquid crystal display apparatuses to which the present invention isapplied.

FIG. 16 is a cross-sectional view showing a solar cell to which thepresent invention is applied.

FIG. 17 is a schematic diagram showing an embodiment of a semiconductorthin film fabricating apparatus used at the time when the presentinvention is applied to fabrication of a solar cell.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described below withreference to the accompanying drawings.

Initially, a first embodiment in which the present invention is appliedto fabrication of a polycrystalline silicon thin film will be describedbelow with reference to the drawings. FIGS. 1(A) and (B) are a plan viewand a sectional view, respectively, showing the crystal structure ofpolycrystalline silicon formed on a glass substrate 1 and a lower baseinsulation film 2 through a method of the present invention. FIGS. 1(C)and (D) are a plan view and a sectional view, respectively, showing thecrystal structure of polycrystalline silicon having an average crystalgrain size of 210 nm fabricated through a conventional lasercrystallization method. The film thickness of the polycrystallinesilicon in both examples is 60 nm. It can be understood from the planestructure of the polycrystalline silicon that is formed in accordancewith the present invention, as shown in FIG. 1(A), that the averagecrystal grain size is as large as 530 nm, and from the cross-sectionalview of FIG. 1(B) that projections are hardly produced at the crystalgrain boundaries. On the other hand, in the polycrystalline silicon thatis formed according to the conventional fabrication method, a projectionhaving a level of height equal to the thickness of the film is producedat the crystal grain boundary. In accordance with the present invention,the size of a crystal grain is defined as the size of a pattern whoseperiphery is surrounded by etched grain boundaries appearing when thesurface of the polycrystalline silicon is treated with SECCO etchingsolution to selectively etching the crystal grain boundaries. Thispattern can be observed using an analysis apparatus, such as a scanningelectron microscope (SEM), an atomic force microscope (AFM), a scanningtunneling microscope (STM) or the like. Further, the grain size of eachcrystal grain is defined by the diameter of a circle having an areaequal to the area of each crystal grain, and the average crystal grainsize 1a of an arbitrary region (an area S) containing m crystal grainsis defined by the following equation. In accordance with the presentinvention, in the case of using a SEM, the area S of the region toobtain an average crystal grain size is defined as the sum of areas ofcrystal grains each of which is photographed at a magnification capableof checking the size of a crystal grain and each of which isphotographed so that the whole grain boundary falls within the field ofview without being cut out of the field of view. In a case where thereare a plurality of fields of view, the area S of the region is definedas the total sum of the sums of areas of crystal grains each of whichfalls within the individual field of view in the whole grain. In thecase of using an AFM or a STM, the area S of the region is defined asthe sum of areas of crystal grains, each whole grain boundary of whichfalls within the arbitrary measuring field.1a=2X((S/m)/π)^(1/2)

In accordance with the present invention, the shape of each crystalgrain is irregular because the polycrystalline silicon is formed bylaser crystallization. However, since the polycrystalline siliconisotropically grows from a generated crystal seed in a directionparallel to the substrate when the melted silicon is crystallized, theshape of the polycrystalline silicon crystal does not become a complexdendrite which can be observed in polycrystalline silicon formed througha solid phase growth method. From observation of crystal grains aftertreating a surface of the polycrystalline silicon with SECCO etchingsolution, it was found that, in the polycrystalline silicon that isformed in accordance with the present invention, at least more than 50%of the crystal grains satisfied the relation Ln≦4πRn. Therein,Rn=(Sn/π)^(1/2), Sn is a surface area of an arbitrary crystal grain, andLn is a circumferential length on the surface of the arbitrary crystalgrain. From the above relation, it can be understood that the volume ofthe crystal grain boundary of the polycrystalline silicon in accordancewith the present invention is smaller than that of the polycrystallinesilicon film prepared through a solid phase growth and composed ofcrystal grains for which the above relation generally does not hold, andthat the polycrystalline silicon formed in accordance with the presentinvention is a high-quality crystalline semiconductor having a smalldefect density over the whole silicon film. Further, from observation ofa cross section of the polycrystalline silicon of the present inventionusing a transmission electron microscope (TEM), it was found that morethan 70% of all the crystal grains continuously extended from aninterface between the semiconductor layer and a base layer to thesemiconductor surface without a discontinuity at some midpoint. Becausethere are few regions in the film where plural crystal grains arestacked up in the direction of the film thickness, the polycrystallinesilicon in accordance with the present invention is a high-qualitypolycrystalline silicon having less defects caused by crystal grainsextending in the lateral direction of the substrate. It was found frommeasurement of the surface of the semiconductor film using an atomicforce microscope (AFM) that the average roughness of the surface was 2nm. Therein, the average roughness of the surface with respect to thepresent invention means an arithmetic average roughness (Ra), and in acase of three-dimensionally analyzing the surface form of thesemiconductor, the average roughness of the surface is a value obtainedby dividing a volume of a portion that is surrounded by a quadricsurface of the surface form and a plane at the average level of thesurface form by the measured area. In accordance with the presentinvention, when an analyzing apparatus that is capable ofthree-dimensionally analyzing the surface form, such as an AFM, ascanning tunneling electron microscope (STM) or the like, is used foranalyzing a measured area, the measured area is an arbitrary region ofthe semiconductor surface having a size larger than an analysis limitarea which is analyzable by the analyzing means. On the other hand, inthe case of analyzing a surface form of the two-dimensionalcross-sectional structure of a semiconductor, the average roughness ofthe surface is a value obtained by dividing an area of a portionsurrounded by a quadric curve of the surface form and a line in theaverage level of the surface form by the measured length, as shown inFIG. 2. The cross-sectional structure can be observed by a photographtaken by a TEM or a high-resolution scanning electron microscope (SEM).Further, the cross-sectional structure can be observed by scanning themeasuring needle of an AFM or an STM once. In accordance with thepresent invention, when an electron microscope, such as an AFM or anSTM, is used as an analyzing means, the region of measured length forcalculating the average roughness is a field of view or plural fields ofview of the electron microscope in a magnitude capable of calculatingthe roughness. When an AFM or STM is used, the measured length is anarbitrary region of the semiconductor surface having a length longerthan an analysis limit length. In accordance with the present invention,the method of calculating the average roughness may employ either amethod of calculating from a three-dimensional form or a method ofcalculating from a cross-sectional form. Letting the average height ofthe plane be the X-Y plane, the longitudinal direction be the Z axis,and the curve of the measured surface form be z=f(x, y), when athree-dimensional form is observed, the average roughness Ra can beexpressed by the following equation:Ra=(1/(Lx·Ly))X∫ ^(Lx) ₀∫^(Ly) ₀ f(x, y)dxdywhere Lx is a measured length in the X direction, and Ly is a measuredlength in the Y direction.

On the other hand, Letting the average height of the line be on X axis,the longitudinal direction be on Y axis, and the curve of the measuredsurface form be y=f(x), when the cross-sectional form is observed, theaverage roughness Ra can be expressed by the following equation:Ra=(1/Lx)X ∫ ^(Lx) ₀ f(x)dx,where Lx is a measured length in the X direction.

Further, from observation of the polycrystalline silicon film inaccordance with the present invention by X-ray diffractometry, it wasfound that the crystal plane of the silicon film was oriented to mainlythe (1. 1. 1) plane. The polycrystalline silicon in accordance with thepresent invention is very useful for an active layer of a thin filmtransistor and a semiconductor layer of a solar cell because the crystalgrain size is as large as 530 nm and the roughness is as even as 2 nm;and, accordingly, the performance of the thin film transistor or thesolar cell can be largely improved by using the polycrystalline silicon.

A method of fabricating the polycrystalline silicon film in accordancewith the present invention will be described below. FIG. 3(A) shows asemiconductor thin film fabricating apparatus used in the processaccording to the present invention, which apparatus comprises asubstrate loading chamber L, an insulation film forming chamber R1, anintrinsic semiconductor film forming chamber R2 and a laser processingchamber LA arranged so as to surround a transfer chamber T, which iscomposed of a substrate transfer robot and an evacuating pump, and therespective chambers are connected to the transfer chamber T. A substratecan be transferred without being exposed to the atmosphere between eachof the film forming chambers and the laser processing chamber throughthe transfer chamber T, in which the vacuum level is maintained higherthan 10-5 torr. The polycrystalline silicon film in accordance with thepresent invention also can be fabricated by an apparatus having a layoutin which a substrate loading chamber L, an insulation film formingchamber R1, an intrinsic semiconductor film forming chamber R2 and alaser processing chamber LA are arranged in line, and a transfer chamberT composed of a substrate transfer robot and an evacuating pump isarranged along the four chambers, as shown in FIG. 3(B).

Initially, a base insulation film 2 having a thickness of 300 nm formedof silicon oxide is formed on a glass substrate 1 while maintaining thesubstrate at a temperature below 350° C. through plasma assistedchemical vapor deposition (PECVD) using tetra-ethyl ortho-silicate(TEOS) and oxygen as raw materials, as shown in FIG. 4(A). As anexample, the glass substrate 1 can be a no-alkali glass substrate havinga softening point lower than 700° C. Next, an amorphous silicon film 3having a thickness of 35 nm and a combined hydrogen concentration in thefilm of 7% is formed while maintaining the substrate at a temperaturebelow 400° C. using mono-silane and hydrogen as raw materials in theintrinsic semiconductor film forming chamber R2, as shown in FIG. 4(B).Next, the glass substrate 1 is transferred from the intrinsicsemiconductor film forming chamber R2 to the laser processing chamber LAthrough the transfer chamber T while maintaining a vacuum. In the laserprocessing chamber LA, the substrate temperature is kept at 350° C.using a heater. Then, as shown in FIG. 4(C), a polycrystalline silicon 5is formed by irradiating a Xe-Cl excimer laser beam 4 having a line beamshape by selecting an irradiation pitch so that the preceding laser beamand the following laser beam are overlapped by 90% with respect to eachother (an irradiation pitch equivalent to performing 10 times of laserirradiation on any position) and increasing the energy density of thelaser beam from 200, 300 to 380 mJ/cm² stepwise. Because the laser isirradiated in multi-steps, the amorphous silicon film 3 isdehydrogenated while the weak energy laser is being irradiated.Therefore, the amorphous silicon film 3 can be crystallized withoutdamaging the film even if the combined hydrogen concentration in thefilm is above 3%. If the combined hydrogen concentration in the film isbelow 10%, a sufficient number of steps of laser irradiation is three orless. Therefore, the fabrication time can be shortened, and,accordingly, the throughput can be improved. After completion of thelaser irradiation, the glass substrate 1 is transferred from the laserprocessing chamber LA to the intrinsic semiconductor film formingchamber R2 through the transfer chamber T while maintaining a vacuum,then, as shown in FIG. 4(D), an amorphous silicon film 3 having athickness of 25 nm and a combined hydrogen concentration in the film of7% is formed while maintaining the substrate at a temperature below 400°C. using moni-silane and hydrogen as raw materials in the intrinsicsemiconductor film forming chamber R2. Next, the glass substrate 1 istransferred from the intrinsic semiconductor film forming chamber R2 tothe laser processing chamber LA through the transfer chamber T whilemaintaining a vacuum. Then, as shown in FIG. 4(E), a large grain sizepolycrystalline silicon 6 is formed by irradiating a Xe-Cl excimer laserbeam 4 having a line beam shape by selecting an irradiation pitch sothat the preceding laser beam and the following laser beam areoverlapped by 90% with respect to each other and by increasing theenergy density of the laser beam from 200, 300 to 520 mJ/cm² stepwise.Thus, fabrication of the polycrystalline silicon is completed. Asdescribed above, according to the present invention, it is possible toform a high-quality polycrystalline silicon film having a film thicknessof 60 nm, an average size of the crystal grains of 530 nm and an averagesurface roughness of 2 nm on an economical glass substrate. Accordingly,the fabrication cost of the high-quality polycrystalline silicon can bereduced.

The laser crystallization mechanism of the present invention will bedescribed below. The inventors have found that a polycrystalline siliconfilm having an average surface roughness below 5 nm and smallprojections and an average crystal grain size above 300 nm could befabricated by irradiating laser light in three steps from low energy tohigh energy on an amorphous silicon film having a film thickness below40 nm. This was determined from the fact that the crystal growth duringlaser irradiation in the third step was mainly crystal growth ofsecondary grain growth by melt-combining of crystal grains having agrain size of 100 to 200 nm which had been produced from isotropicgrowth by melting and solidifying by the laser irradiation up to thesecond step, and not that the crystal growth during laser irradiation inthe third step was crystal growth of melted silicon in the lateraldirection which produced the projections. The amorphous silicon 3 of thefirst layer crystallized first is a film as thin as 35 nm, and thepolycrystalline silicon 5 of the first layer, having an averageroughness in the surface of 5 nm and an average crystal grain size of400 nm, is formed by the laser crystallization method of three stepirradiation in accordance with the present invention. When the lasercrystallization for the second layer is performed, the film of the firstlayer is already crystallized to polycrystalline silicon, and,consequently, the melting point becomes higher than that of amorphoussilicon. Therefore, the melted zone of the polycrystalline silicon 5 ofthe first layer during the laser crystallization for a second layer isonly in the vicinity of the boundary zone with the silicon in the secondlayer. The film thickness of the amorphous silicon in the second layeris 25 nm, and, accordingly, the silicon melted during the lasercrystallization for the second layer is the whole silicon in the secondlayer and only the very thin zone in the upper layer of the silicon ofthe first layer. Therefore, the thickness of the silicon melted duringthe second laser crystallization is thinner than the thickness of thesilicon melted during the first laser crystallization. It is believedthat the grain size of the semiconductor thin film is likely to growlarger by the secondary grain growth caused by melt-combining with anadjacent crystal grain by plural times of laser irradiation, as thethickness of the semiconductor thin film becomes thinner. Thetheoretical explanation is reported in the Applied Physics Letter, Vol.44, No. 6, 19894, p602. The non-melted polycrystalline silicon 5 in thelower layer is heated up to 1000° C. or more by heat conduction bycontact with the melted silicon and absorption of the laser light. Sincethis portion serves as a heat reservoir when the melted silicon in theupper layer is crystallized, the cooling speed is slowed down, and,accordingly, the crystallization speed is decreased to form the largegrain sized polycrystalline silicon 6 having an average crystal grainsize of 530 nm. This grain size is larger than the average crystal grainsize of 210 nm, which is obtained from laser-crystallizing a singleamorphous silicon layer having a thickness of 60 nm, which is equal tothe sum of the thickness of the silicon films of the first layer and thesecond layer, similar to the conventional method. In order to improvethe effect of the heat reservoir, it is preferable that the thickness ofthe silicon of the first layer is set to a value larger than thethickness of the silicon in the second layer. Further, since thesecondary grain growth obtained by melt-combining with the adjacentcrystal grain occurs when the semiconductor layer is in a hightemperature state above 600° C. even if the semiconductor layer is notmelted, growth of the crystal grains in the heated polycrystallinesilicon of the first layer by the secondary grain growth and themelt-combination between the growing crystal grains of the first layerand the crystal grains under growing by solid crystallization occur atthe same time. As a result, in the produced large grain sizedpolycrystalline silicon 6, at least more than 70% of the crystal grainscontinuously extend from the interface between the semiconductor layerand a base insulation layer 2 to the semiconductor surface withoutdiscontinuity at some midpoint. Therefore, since the ratio of the regionwhere the crystal grains overlap in the film-thickness direction isdecreased, the number of defects caused by the crystal grain boundariesin the polycrystalline silicon become small, and, accordingly, thecharacteristic of the semiconductor can be improved. Further, since themechanism of crystal grain growth obtained in the first and the secondlayers is the secondary grain growth by melt-combining of the crystalgrains, as described above, which is not growth in the lateral directionwhich tends to produce a projection on the surface, the height of theprojection produced during laser crystallization becomes lower than theheight of the projection which is obtained from laser-crystallizing asingle amorphous silicon layer having a thickness equal to the sum ofthe thickness of the silicon films of the first layer and the secondlayer, similar to the conventional method, and, consequently, it ispossible to suppress the occurrence of the unevenness on thepolycrystalline silicon surface. Furthermore, even if the projectionsare formed in the polycrystalline silicon of the first layer, theprojections are melted first in the polycrystalline silicon of the firstlayer when the amorphous silicon of the second layer is lasercrystallized, because the crystal quality of the projections is bad andthe melting point is as low as that of amorphous silicon. Therefore, theprojections on the surface are suppressed over the whole polycrystallinesilicon film so as to make the surface even. As a result, the averageroughness of the surface of the polycrystalline silicon fabricatedaccording to the present invention becomes 2 nm, and the greaterevenness increases the applicability of the polycrystalline silicon tovarious kinds of semiconductor devices, such as a thin film transistorand so on, as will be described in connection with the followingembodiments. The film thickness of the polycrystalline silicon 6 in theabove-described embodiment is 60 nm. However, if the film thickness ofthe polycrystalline silicon 6 is below 40 nm, a small amount of metallicions and oxygen, nitrogen, and a carbon impurity contained in the baseglass substrate 1 and the base insulator film 2 are diffused into thepolycrystalline silicon 6 by heat during laser irradiation to decreasethe quality of the film, not only in the lower portion, but also in theupper portion of the polycrystalline silicon. From this viewpoint, athickness of 60 nm is preferable. It is advantageous for controlling thefilm thickness and preferable from the viewpoint of application to asemiconductor device, such as a thin film transistor, that the filmthickness of the polycrystalline silicon 6 is above 40 nm. On the otherhand, in regard to the upper limit of the film thickness, a filmthickness below 100 nm is preferable from the viewpoint of the abilityto cover silicon steps of metal or insulation to be deposited on theisland-shaped polycrystalline silicon 6 when a semiconductor device,such as a thin film transistor or a liquid crystal display apparatus, isfabricated. This upper limit is not applied in a case where thepolycrystalline silicon is applied to a semiconductor apparatusrequiring a thick semiconductor film such as a solar cell or the like.In accordance with the present invention, by repeating the steps offorming the amorphous silicon film and crystallizing the amorphoussilicon film by irradiating laser light at least two times, apolycrystalline silicon having a film thickness above 40 nm and below100 nm, an even surface and a large crystal grain size can be easilyfabricated. The film thickness of the amorphous silicon of the firstlayer and the film thickness of the amorphous silicon of the secondlayer are set to 35 nm and 25 nm in the above embodiment, respectively,but are not fixed to these values. If the film thickness of the firstlayer is thicker than the thickness of the second layer and the totalthickness of the first and the second layers is above 40 nm, the valuesof film thickness may be varied depending on the film quality of eachsemiconductor and the kind of semiconductor apparatus to which thesemiconductor is applied. In accordance with the present invention,since the substrate is transferred from the film forming chamber to thelaser processing chamber while maintaining a vacuum, no concentrationpeak of silicon oxide will exist on the surface of the polycrystallinesilicon 5 of the first layer during film-forming of the amorphoussilicon of the second layer, and, consequently, the characteristics ofthe polycrystalline silicon are not deteriorated by diffusion of oxygenatoms in the melted silicon during laser-crystallizing of the amorphoussilicon of the second layer. Of course, no concentration peak of oxygenexists in the polycrystalline silicon. In addition, since no siliconoxide layer exists on the surface of the polycrystalline silicon 5 ofthe second layer, and, consequently, oxygen atoms do not diffuse fromthe surface to the inside during being melted at the time of lasercrystallization, the characteristic of the polycrystalline silicon canbe improved. As described above, according to the present invention, itis possible to form a polycrystalline silicon film having a filmthickness above 40 nm, an even surface having an average surfaceroughness of 2 nm, an average size of the crystal grains above 500 nmand a good semiconductor characteristic with less oxygen concentrationin the film on an economical glass substrate.

In the above embodiment, although the silicon oxide base film under thesilicon film is an even film, the shape of the base film may bepatterned through the photo-lithography method and the wet or dryetching method. As shown in FIGS. 5(A) and 5(B), a 10 nm step wasprovided on a base film 2 made of silicon oxide, and the steps offorming of an amorphous silicon film by the PECVD process and lasercrystallization was repeated twice through the same process as describedabove. In this case, the inventors found that a crystal grain boundarywas produced at the step portion, as shown in FIG. 5(C). Further, it wasalso found that the crystal grains in contact with the step on the baseinsulation film located at the lower position of the step were orientedmainly in the (1, 1, 0) plane or the (1, 0, 0) plane. Although the causeof occurrence of these phenomena is unknown, the position producingdeterioration at the crystal grain boundary of the electriccharacteristic of the polycrystalline silicon could be controlled bychanging the shape of the base insulator film. By doing so, it waspossible to obtain a high-quality polycrystalline silicon having anaverage crystal grain size above 500 nm and an average surface roughnessof 2 nm, in which at least a part of the crystal grain boundaries werepositioned.

Further, in accordance with the present invention, since the substrateis heated during laser crystallization, the heating time of thesubstrate after transferred between the laser crystallization processand the semiconductor thin film forming process can be shortened.Therefore, the throughput of the fabrication of the crystallinesemiconductor thin film can be improved. Further, since the coolingspeed of the semiconductor melted by laser irradiation can be made slowby heating the substrate during laser crystallization, the crystal grainsize of the crystalline semiconductor thin film can be made large, and,consequently, the quality of the crystal can be improved. In the presentembodiment, although the film forming temperature of the base insulationfilm, the film forming temperature of the amorphous silicon and the basetemperature during laser crystallization are set to 350° C., 400° C. and350° C., respectively, it is possible that these temperatures need notbe fixed, but may be varied depending on the film quality of theinsulation film or the semiconductor or the performance of the targetsemiconductor apparatus. However, when a glass substrate is used for thesubstrate and silicon is used for the semiconductor, these temperaturesare preferably set to a value between 200° C. and 500° C. In theapparatus for fabricating the polycrystalline silicon used in thepresent embodiment, each of the film forming chambers and the lasercrystallizing chamber and the substrate loading chamber are connectedwith the transfer chamber T, which is composed of a substrate transferrobot and an evacuating pump, as shown in FIGS. 3(A) and 3(B). However,the polycrystalline silicon in accordance with the present invention canbe fabricated by an in-line type semiconductor fabricating apparatus inwhich each of the film forming parts, a laser crystallizing part and atransfer part are arranged in a single chamber, as shown in FIG. 6. Byusing an in-line type semiconductor fabricating apparatus which is lowin manufacturing cost, the cost of equipment can be suppressed, and,consequently, the semiconductor apparatus can be fabricated with a lowcost. In the present, embodiment the substrate is transferred bymaintaining the transfer system at a high vacuum below 10⁻⁵ torr.However, the substrate may be transferred under an atmosphere of aninert gas, such as nitrogen, helium, neon, argon or the like, becausethe present invention requires that a natural oxide film not be formedon the silicon surface after forming the amorphous silicon film or afterlaser crystallizing the amorphous silicon. In this regard, the transferunder an atmosphere of an inert gas can attain the same effect as thetransfer in a vacuum.

In the above embodiment, the Xe—Cl excimer laser is used forcrystallizing the silicon film. However, various kinds of excimerlasers, such as a Kr—F excimer laser, an Ar—F excimer laser or the like,or a continuous-wave laser, such as an Ar ion laser and the like, or anenergy beam, such as an electron beam or the like, may be used forcrystallizing regardless of the kind of device in principle, if it cancrystallize the silicon film.

Further, in the above embodiment, although the present invention isapplied to amorphous silicon, the present invention can be applied tofine-crystalline silicon, polycrystalline silicon, impurity-dopedsilicon, silicon-germanium (SiGe), germanium (Ge) and so on, regardlessof the kind of semiconductor, if it can be crystallized by heating usinga laser. Further, in the above embodiment, the film forming apparatus isof the plasma assisted chemical vapor deposition type. However, alow-pressure chemical vapor deposition type (LPCVD), a sputtering type,an ion beam type, an atom beam type, a molecule beam type, a spin-coattype or a vapor deposition type film forming apparatus may be employedregardless of the type, if it can be connected to the transfer chamber Tand can transfer the substrate between the transfer chamber and thelaser processing chamber LA without exposing the substrate to theatmosphere or while maintaining a vacuum. Furthermore, in the aboveembodiment, the substrate used is made of glass. However, a substratemade of an organic resin, such as a substrate made of polyimide,polyamide or polyester can be used if the film forming temperatures ofthe base insulator film 2 and the amorphous silicon 3 are reduced to avalue lower than 300° C.

The second embodiment of fabrication of a copalanar type thin filmtransistor to which the present invention is applied will be describedbelow.

As shown in FIG. 7(A), a large grain size polycrystalline silicon 6,which is selected to be an active layer of a thin film transistor, isformed on a glass substrate 1 having a base insulator film 2 using amethod similar to that in the first embodiment. As shown in FIG. 7(B),after forming the polycrystalline silicon 6 into an island-shape throughthe photo-lithographic method and the dry etching method, a gateinsulation film 7, having a film thickness of 100 nm, is formed by theplasma assisted chemical vapor deposition method using TEOS and oxygenas raw materials. Then, after forming a niobium (Nb) film having a filmthickness of 250 nm through the sputtering method, a gate electrode 8 isformed by processing the niobium film using the photo-lithographicmethod and the dry etching method. Next, as shown in FIG. 7(C), afterimplanting the impurity phosphorus (P) into a region of the active layernot overlapping with the gate electrode 8 using the ion implantationmethod, a source region 9 and a drain region 10 are formed by activatingthe impurity by heating the substrate at 500° C. for 2 hours using anelectric furnace. Then, as shown in FIG. 7(D), after forming aninter-layer insulation film 11 having a film thickness of 500 nm throughthe plasma assisted chemical vapor deposition method using TEOS andoxygen as raw materials, contact holes are formed through thephoto-lithographic method and the dry etching method. Finally, afterforming a chromium-molybdenum alloy (Cr—Mo) film having a film thicknessof 500 nm through the sputtering method, a gate electrode, a sourceelectrode 12 and a drain electrode 13 are processed through thephoto-lithographic method and the wet etching method. Thus, the thinfilm transistor is completed.

By using polycrystalline silicon having a large crystal grain size of530 nm and an average surface roughness of 2 nm for the active layer, itwas possible to fabricate a high-performance thin film transistor havinga mobility of 300 cm²/V·s which is largely improved compared to themobility of 140 cm2/V·s of a thin film transistor fabricated through theconventional method.

FIG. 8 is a graph which compares the gate voltage versus drain currentcharacteristics of a polycrystalline silicon thin film transistor inaccordance with the present invention with those of a thin filmtransistor having an active layer made of a polycrystalline siliconwhich is fabricated in the conventional manner, in which an amorphoussilicon film having a film thickness of 60 nm formed through the plasmaassisted chemical deposition method is heated at 450° C. under anitrogen atmosphere to perform dehydrogenating treatment, and then theamorphous silicon is laser crystallized. It is clear from FIG. 8 thatthe ON current in the polycrystalline silicon thin film transistor ofthe present invention is increased by the effects of an increase in thecrystal grain size and a decrease in the roughness (projections) of theactive layer film. Further, because the electric field concentration inthe projection is suppressed by the even surface of the active layer ofthe polycrystalline silicon 6 in accordance with the present invention,the dielectric breakdown voltage of the gate insulator film 7 is largelyincreased from 4 MV/cm to 7 MV/cm to improve the withstand voltagecharacteristic, and the reliability relative to hot carriers is alsoimproved. Furthermore, since the thickness of the gate insulation filmcan be made thinner, that is, below 80 nm, compared to that in theconventional method, the thin film transistor can be driven by a lowervoltage, and, accordingly, the power consumption of a semiconductorapparatus using the thin film transistor can be reduced. By using thepolycrystalline silicon in accordance with the present invention in anactive layer of a thin film transistor, the film thickness of the gateinsulation film and the film thickness of the active layer can be set sothat a ratio of the film thickness of the gate insulation film to thefilm thickness of the active layer becomes a value smaller than 8/6.Further, by optimizing the film quality of the gate insulation film, thethickness of the gate insulation film can be made thinner, and,accordingly, the thin film transistor can be driven by a lower consumedpower.

Although application of the present invention to a coplanar type thinfilm transistor has been described in the above embodiment, the presentinvention can be applied to an inverse stagger type or a normal staggertype thin film transistor regardless of the type of the thin filmtransistor.

A third embodiment of an active matrix type liquid crystal displayapparatus to which the thin film transistors in accordance with thepresent invention are applied will be described below.

FIG. 9 is a circuit diagram showing the construction of the embodimentof the active matrix type liquid crystal display apparatus in accordancewith the present invention. Referring to the figure, a thin filmtransistor (TFT) is provided in each of a plurality of liquid cells (LC)arranged in the form of a matrix, and each of the liquid cells is drivenby switching operation of the TFT. Therein, a gate voltage issequentially applied to gate lines G1 to GM of electrodes commonly ledout from individual gates of the TFTs aligned in the lateral directionon a glass substrate 1 to switch on the gates from gate line to gateline. On the other hand, a data voltage for each switched-on gate lineis sequentially applied to drain lines D1 to DN of electrodes commonlyled out from individual drains of the TFTs aligned in the perpendiculardirection to be supplied to each of the liquid cells.

FIG. 10 is a plan view showing the plane structure of one pixel composedof one liquid cell and one TFT. Further, FIG. 11 shows thecross-sectional structure along the fine X-X′ of FIG. 10. The pixel iscomposed of a TFT formed near a cross point of the drain wire D and thegate wire G and the liquid cell LC connected to the TFT through a sourceelectrode 12. The sectional structure of the TFT is nearly equal to thatof the second embodiment. The structure can be obtained through thefabricating method described in the second embodiment, but pointsdifferent from the above-described process are as follows. Ionimplantation at the time of forming the source-drain region isseparately performed twice, and a locally doped drain (LDD) regionhaving a lower impurity concentration is provided in the source-drainregion in contact with the active layer in order to reduce the OFFcurrent. Further, the gate wire G is formed by processing film-formingand etching together with the gate electrode 8. An inter-layer insulatorfilm 14 made of SiN is formed after forming the source and the drainelectrodes 12, 13. After processing this workpiece to bore a contacthole to the source electrode 12, an indium-tin oxide film (ITO) isformed and patterned to form a pixel electrode 15. Parts other than theTFT, such as the liquid crystal, will be described below.

TN type liquid crystal 16 is filled between the glass substrate 1 havingthe TFT formed thereon and a glass substrate 17 (an opposite substrate)opposite to the glass substrate 1. A black matrix 18 for cutting offunnecessary light and an opposite electrode 19 made of ITO are formed onthe opposite substrate. The liquid crystal is driven by a voltagebetween the opposite electrode 19 of the opposite substrate 17 and thepixel electrode 15 of the TFT substrate, and an image is displayed onthe matrix of the pixels by changing the display brightnesspixel-by-pixel. A polarizing plate 20 for polarizing light is attachedonto each of the glass substrates 1, 17. By orientating the polarizationaxes of these two polarizing plates in orthogonal or parallel directionsrelative to each other, a normally black display mode or a normallywhite display mode is obtained, respectively. Polarizing films 21 forpolarizing the liquid crystal are applied onto the surfaces in contactwith the liquid crystal, that is, onto the surface of the inter-layerinsulation film 14 and the pixel electrode in the glass substrate 1 sideand onto the surface of the opposite electrode 19 in the oppositesubstrate 17 side. After applying the polarizing film, the surface ofthe polarizing film is processed by the lapping method to add anisotropyfor orientating the liquid crystal molecules to the polarizing film. Aback light serving as a light source is placed on the substrate 1 sideto give brightness to the display.

By using the TFTs fabricated as described above in accordance with thepresent invention for the driving elements of the display part pixels ofthe active matrix type liquid crystal display apparatus, the TFT can bemade small because the grain size of the active layer polycrystallinesilicon is large and the mobility is large. Therefore, the opening ratioof the pixel is increased, and, accordingly, the power of the back lightcan be reduced by that amount, whereby a reduction in the powerconsumption of the liquid crystal display apparatus can be attained.Further, since the surface of the TFT active layer is made even and theprojections are eliminated, the thickness of the gate insulation filmcan be made thinner than 80 nm. Therefore, the TFT can be driven by alow voltage, and, accordingly, low power consumption in the liquidcrystal display apparatus can be attained. Furthermore, since thesurface is made even and, accordingly, the characteristic inside thepolycrystalline silicon is made uniform, display defects caused bynon-uniformity of the characteristic of the polycrystalline silicon TFTcan be eliminated.

As shown by FIG. 12(A) and FIG. 12(B), which is a cross-sectional viewtaken on the plane of the dashed line X-X′ in FIG. 12(A), in a casewhere a signal storage capacitor of the pixel is formed by sandwiching asilicon oxide film in the same layer of the gate insulation film with ann-type polycrystalline silicon formed by doping phosphorus into thepolycrystalline silicon in the same layer of the polycrystalline silicon6 of the active layer and the common wire formed of a metallic thin filmin the same layer of the gate wire, if the polycrystalline silicon inaccordance with the present invention is used, the film thickness of thesilicon oxide film can be made thin because the unevenness on thesurface is very small and, consequently, there is no possibility ofoccurrence of dielectric breakdown caused by electric fieldconcentration at a projection. Because of the distance between theelectrodes, the capacitance of the signal storage capacitor isincreased, and, consequently, the area of the electrode can be reducedand the opening ratio of the pixel can be increased. Therefore, lowpower consumption of the liquid crystal display apparatus can beattained by reducing the brightness of the back light.

Further, by constructing the peripheral circuit for driving the displaypart using the TFTs in accordance with the present invention, as shownin FIG. 13(A), a small-frame size and low-cost liquid crystal displayapparatus incorporating a peripheral circuit can be manufactured byeliminating the driver LSI. Further, as shown in FIG. 13(B), byincorporating not only the display part driving circuit, but also amemory, a micro processing unit (MPU) and another information processingunit formed through micro-processing in the periphery of the displaypart, an advanced liquid crystal display apparatus can be obtained byadding a communication function and a calculating function to the liquidcrystal display apparatus.

Furthermore, after crystallizing an amorphous silicon in the first layeras shown in FIG. 14(A), an amorphous silicon film 3 in the second layeris formed without exposing the atmosphere, as shown in FIG. 14(B), andthen a part of the region of the amorphous silicon 3 in the second layermay be intentionally left in the amorphous state without irradiatinglaser light on that region, as shown in FIG. 14(C). Then, as shown inFIG. 15(A), in the polycrystalline silicon/amorphous silicon regionwhere the amorphous silicon in the second layer has been not irradiated,a display, part is provided to form a coplanar type TFT. In thepolycrystalline silicon/polycrystalline silicon region where theamorphous silicon in the second layer has been laser crystallized, aperipheral circuit part is provided to form a copalanar type TFT of CMOSstructure for driving the circuit. By doing so, because the active layerof the display part TFT becomes high resistance amorphous silicon, theOFF current is reduced, and, accordingly, there is no need to form anoffset structure or a lightly doped drain (LDD) structure, which isnecessary when the display part TFT is constructed using thepolycrystalline silicon TFT. Therefore, the number of processes forfabricating the peripheral circuit incorporating the liquid crystaldisplay apparatus is reduced by that amount, and, consequently, thethroughput of the fabrication is improved and the cost is reduced.

The same effect can be obtained in a case where the TFT structure is ofan inverse stagger type because a high resistance amorphous siliconexists between the polycrystalline silicon of the active layer and thesource-drain region, as shown in FIG. 15(B). In contrast to the secondembodiment, in fabricating the polycrystalline silicon TFT describedwith reference to FIGS. 14(A) to (C) and FIGS. 15(A) and 15(B), when thesource-drain region is formed, the impurity activation is performedusing laser activation in order to prevent the amorphous region frombeing deteriorated and not using activation of thermal annealing usingan electric furnace.

The present invention can be also applied to the formation of asemiconductor layer in a solar cell which is composed of a p-type,n-type and intrinsic polycrystalline silicon layers, as shown in FIG.16. In accordance with the present invention, since the large grainsized and even polycrystalline semiconductor can be fabricated even ifthe thickness is increased, a solar cell having a high solar lightconverting efficiency above 15% can be fabricated. The solar cell can befabricated with the fabricating apparatus shown in FIG. 3(A), 3(B) orFIG. 6. However, in a case where the substrate is a long continuoussubstrate made of stainless steel or aluminum, the solar cell can befabricated using an inline type fabricating apparatus as shown in FIG.17. By using the inline type fabricating apparatus, the solar cell canbe fabricated with a low cost. As described above, the present inventioncan be applied to all the semiconductor apparatuses usingpolycrystalline semiconductors in addition to thin film transistors.

According to the present invention, it is possible to fabricate ahigh-quality crystalline semiconductor having a film thickness above 40nm, an average size of the crystal grains larger than 500 nm and anaverage roughness of the surface smaller than 5 nm on an economicalglass substrate. Further, it is possible to improve the performance ofthe semiconductor apparatus using a semiconductor, such as a thin filmtransistor, a solar cell and an active matrix type liquid crystaldisplay apparatus.

1. A method of fabricating a semiconductor thin film, the methodcomprises the steps of: forming an insulation film on a glass substrate;forming a semiconductor thin film on the insulation film; andcontinuously following the semiconductor thin film forming,crystallizing the formed semiconductor thin film by irradiating laserlight without exposing to atmosphere, wherein the set of forming thesemiconductor thin film and crystallizing the semiconductor thin film byirradiating laser light is repeated at least two times, and a method ofirradiating laser light in each of the sets is that energy of the laserlight is increased in steps from scanning of weak energy laser light toscanning of strong energy laser light.
 2. A method of fabricating asemiconductor thin film according to claim 1, wherein the formedsemiconductor thin film is silicon having a concentration of combinedhydrogen in the film less than 10%.
 3. A method of fabricating asemiconductor thin film according to claim 1, wherein temperature of thesubstrate during the laser crystallization is within a range of 200° C.to 500° C.
 4. A method of fabricating a semiconductor thin filmaccording to claim 1, wherein the glass substrate used is a no-alkaliglass and has a softening point lower than 700° C.
 5. A method offabricating a semiconductor apparatus containing a thin film transistor,wherein the method of fabricating the semiconductor thin film accordingto claim 1 is applied to fabrication of an active layer of the thin filmtransistor.
 6. A method of fabricating a semiconductor apparatuscontaining an active matrix type liquid crystal display apparatus inwhich a thin film transistor is used as a drive element in a pixel andone of electrodes of a signal storage capacitor in the pixel is formedof a semiconductor thin film in the same layer as an active layer of thethin film transistor, wherein a no-alkali glass having a softening pointlower than 700° C. is used for a supporting substrate, and the method offabricating the semiconductor thin film according to claim 1 is appliedto fabrication of the semiconductor thin film forming the one ofelectrodes of the signal storage capacitor in the pixel.
 7. A method offabricating a semiconductor thin film or a semiconductor apparatusaccording to claim 1, wherein the semiconductor thin film or thesemiconductor apparatus is fabricated using an apparatus in which atleast a film forming apparatus for forming a semiconductor thin film anda laser crystallizing apparatus are connected to each other by atransfer apparatus having an evacuating apparatus.
 8. A method offabricating a semiconductor thin film or a semiconductor apparatusaccording to claim 1, wherein the semiconductor thin film or thesemiconductor apparatus is fabricated using an apparatus in which atleast a film forming apparatus for forming a semiconductor thin film, afilm forming apparatus for forming an insulation film and a lasercrystallizing apparatus are connected to each other by transferapparatuses each having an evacuating apparatus.
 9. A method offabricating a semiconductor thin film or a semiconductor apparatusaccording to any one of claims 7 and 8, wherein the atmosphere of thetransfer apparatus is maintained in a vacuum below 10⁻⁵ torr or anatmosphere of an inert gas such as nitrogen, helium, neon, argon or thelike.
 10. A method of fabricating a semiconductor thin film or asemiconductor apparatus according to claim 1, wherein the semiconductorthin film or the semiconductor apparatus is fabricated using an in-linetype apparatus in which at least a film forming portion for forming asemiconductor thin film, a laser crystallizing portion and a transferportion are placed in a single chamber.